1. Field of the Art
The present invention relates to a porous ceramic filter or filter medium, and a process of producing the same, and more particularly to such a ceramic filter of a planar shape having a filtering layer with a uniform thickness and with a porous structure which is protected against clogging, and a process by which such a planar ceramic filter is produced.
2. Related Art Statement
There is known a multiple-layer ceramic filter medium (hereinafter simply called "ceramic filter") which consists of a porous support body of a tubular shape formed of a ceramic material and having a high mechanical strength, and at least one filtering layer formed of a material similar to that of the porous support body and having minute or macro pores. The filtering layer or layers is/are formed integrally on an inner or outer surface of the porous support body. This type of ceramic filter has a relatively low resistance to a flow of a liquid to be filtered, and is very effective in separating fine solid particles from a large amount of flow of the liquid. The ceramic filter is produced, for example, by a process which comprises the steps of: preparing a suspension which contains particles of a suitable material of which a filtering layer is formed; holding one surface of a porous support in contact with the prepared suspension, thereby forming a thin unfired layer of the particles deposited on the surface of the porous support body; and firing the unfired layer into the filtering layer. Another process is disclosed in Japanese Patent No. 1066935 (Japanese Patent Publication No. 56-8643 in 1981). This process includes the steps of: preparing a slurry for forming the filtering layer; introducing the slurry into the tubular porous support body; rotating the support body so that the slurry is spread over the inner surface of the support body, by a centrifugal force, so that a layer of the slurry adheres to the inner surface of the support body; and firing the porous support body with the layer of the slurry on its inner surface. An alternative process is known according to U.S. Pat. No. 4,356,215. This process includes the steps of: preparing two masses of coating suspension, i.e., a first slip coating liquid and a second slip coating liquid, which contain mineral particles having different sizes, respectively; applying the first slip coating liquid to a surface of the porous support body, to form a first deposited layer, heating and drying the first deposited layer, and subsequently firing the porous support body with the first deposited layer; and applying the second slip coating liquid to the fired first deposited layer to form a second deposited layer, heating and drying the second deposited layer, and finally firing the porous support with the first and second deposited layers.